If the M103 oil filter mounting won't fit even without a filter there's always THIS
as recommended to me by Carat 3.6 when I had a similar problem getting an M103 engine into a W114 as detailed HERE If you need this plate you would need to use a different filter with a built in oil-pressure bypass valve to replace the one in the M103 filter mount.
Yeah, it's a M20 confirmed when the remote oil filter kit I ordered turned up with the incorrect adaptor and I had to argue with summit racing. It's my understanding that the correct thread is crawling the globe to me. The unit that turned up had a 3/4 thread for the filter.
I decided to get the starter installed - test that out and get onto the next steps. In my extended period of not working on the car, the engine decided it no longer wished to rotate. bugge.r
Spark plugs out, ATF down the bores, many days of walking out to the car, trying to see if it's free(er), camshafts loosened to reduce the load on the crankshaft nut and success! I now can get the engine to turn over on the starter on a jump pack!
I am now waiting for camshaft bolts to replace the loosened off ones. I believe they are a single use bolt - or perhaps they are not, but 2 of the removed bolts had clearly stretched and there was no chance they were going back in!
I also could see that the M103 housing for the oil filter leaks. The mating face is warped. Good thing I forgot I had it and ordered another one off ebay a few weeks back.